Plain text in email doesn't have size settings. The displayed size is dependent on the settings of whatever software is used to see it.

If your signature uses HTML then you can include font tags to set its size, face, weight, style etc. But you need to learn HTML markup to do it this way. Better to create your signature as if it were a message, save it as an HTML file, and nominate that file in your account settings. Or you could copy and paste the file's content into the signature box in Account Settings.

I'm using TBird. When I open a new blank message, my sig is displayed, in greyed, very small font. Much smaller than the font size of the message content which I type.

Seems to me the sig should be the same size as the message, since I have typed the sig into the box in the Account Settings, and not checked the "HTML" box.

Unless maybe my message is being rendered in HTML?

Have you checked how your signature appears in another email client or a webmail site?

Thunderbird had for a long time chosen to display signatures in a grey font, and some time ago it caused much annoyance when it switched to using alpha channel transparency to implement the grey effect, as this also affected embedded images and logos and suchlike.

Many of us now have a bit of css style coding to disable the transparency thing in both incoming and outgoing messages.

I wasn't aware that the font size was also being tweaked. I've never noticed this happening with signatures myself, but maybe because I have the minimum font size set to something close to that used for normal font size.

So, I could say: "rest assured, it's a Thunderbird thing and won't affect most (many?) other users", but OTOH, while it's good to think that Thunderbird is mostly wysiwyg, it's disturbing to know that certain features such as this are not handled consistently.

Search, and around the web you'll find articles like this:

http://www.jhitsolutions.co.uk/thunderbird-signature-appearing-grey-read/

but I note that this specifically addresses HTML formatted signatures, so there is, for me, still a puzzle as to why plain text signatures are being altered.

I should add that if your message is composed using html, then the signature will be treated as if it were html also, and this can make a mess since html parsers don't recognize white space characters such as carriage returns, linefeeds or tabs as significant, and therefore a multi-line plain text sig usually rolls up into a single long line when rendered in html.

The Signature Switch add-on has a neat solution for this; it allows you to have both a plain and a fancy signature in the same file, the appropriate one being chosen automatically at compose time, thus:

plain sig = html fancy sig

This is for me one of those "this is so fundamental it ought to be built-in" issues. ;-)

However, you'll see that Signature Switch annoys the top-posting brigade, since it uses the dash-dash-space convention to identify the break between text and signature and so accordingly puts the signature at the bottom.

If you don't use dash-dash-space then Thunderbird won't know where the signature is so won't take liberties with it, and you can place it where you want it. Here is where a different approach using Clippings or QuickText might work better for you than using the provided signature tool. Or you could investigate the effect of suppressing the dash-dash-space delimiter.

http://kb.mozillazine.org/Signatures_(Thunderbird)#Removing_the_signature_separator

I'm not sure that my experience supports the statement in that article that html and plain text signatures are largely interoperable. YMMV.

Personally I use my full-blown company-mandated, ugly and graphic-laden signature only on first contact (either my initiating message, or my first reply) and something a great deal more succinct on follow-up messages. So I end up using a pre-defined signature only very infrequently. In most cases, "Regards, Chris" is just fine, easy to type longhand and so and doesn't justify the effort of a full blown signature or Clippings entry.